This invention relates to position sensing or detecting devices for providing a feedback signal proportional to the relative movement between two mechanical elements and which is indicative of the relative position of two such elements. In particular, the invention relates to position detecting devices responsive to the relative linear movement of mechanical elements.
There are many applications in which it is required to provide a signal indicative of the relative movement between mechanical elements. A typical example is a valve actuator having a piston or rod with a certain operating stroke. The position of the valve is typically controlled by means of an electrical signal provided to an electro-mechanical operator within the actuator which controls the movement of the piston rod. Heretofore, the desired electrical feedback signal indicative of piston position, and consequently valve position, has been provided by means of a linear potentiometer associated with the piston or rod.
There are certain problems involved in the use of such potentiometers which adversely affect the overall efficiency of the actuator. Problems are frequently encountered in maintaining satisfactory wiper contact in the potentiometer. In addition, depending on the environment in which the device is operating, substantial reliability problems may be encountered due to atmospheric contamination resulting from moisture, chemicals, corrosion and the like. Such problems substantially increase maintenance costs. Linear potentiometers are also comparatively expensive due to the high costs of precious metals required and the relative difficulty in manufacturing as compared with circular potentiometers.
Further, the linear potentiometers typically must have an effective length equal to the stroke of the piston or rod. This requirement of a one to one relationship often necessitates a size of potentiometer which presents problems from the standpoint of space limitations in certain applications.
It is thus desirable to provide a relatively low-cost position detector which eliminates or substantially reduces the problems heretofore encountered in employing linear potentiometers to provide the required position feedback signal.